1. Field of the Invention
The present invention relates to a duplex metal pipe for damping which is used, for instance, at a portion of an exhaust system of an automobile where vibrations occur and displays a performance of damping the vibrations.
2. Description of the Related Art
In a place of the occurrence of vibrations, such as an exhaust pipe of an automobile, vibrations of an engine, or those resulting from the pulsation of exhaust gas pressure are transmitted to a pipe, and are emitted as noise in the form of the vibration of the pipe itself. To reduce the amounts of vibration and resultant noise, there are known a hollow duplex pipe in which a sufficient space is formed between an inner pipe and an outer pipe and a composite duplex pipe in which a sound-arresting material formed of glass fibers, asbestos, ceramics is filled in the gap between an inner pipe and an outer pipe.
However, with the conventional hollow duplex pipe is not very effective with respect to radiating noise caused by the transmission of engine vibrations although it is effective with respect to radiating noise caused by the pulsation of exhaust gas pressure. In addition, with the composite duplex pipe in which a sound arresting material is filled in an intermediate layer, there has been a problem in that the filler is denatured due to the effect of the heat of exhaust gases and its effect is caused to decline appreciably.